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Grégory Lucas1 – Solymosi József2 – Lénart Csaba3

USING HYPERSPECTRAL IMAGING IN NUCLEAR RADIATION

AERIAL RECONNAISSANCE? A PRELIMINARY STUDY45

This article aims at exploring the potential of hyperspectral imaging in order to develop further the Hungarian

aerial nuclear reconnaissance system. First a description about the theoretical basis of ionizing radiations is

provided. The different types of radiations, their characteristics, penetration range and effect on matter are

presented. Then a critical analysis is done on the Hungarian nuclear reconnaissance system based on the detection

capacities and operational implementation criteria. The last part explores the potential of hyperspectral imaging

technology, its added values and the different possibilities envisaged for its application.

HIPERSPEKTRÁLIS KÉPALKOTÁS ALKALMAZÁSA A NUKLEÁRIS SUGÁRZÁS LÉGI

FELDERÍTÉSÉBEN? ELŐZETES TANULMÁNY

A cikk célja a hiperspektrális képfelvételezés potenciáljának felmérése a magyar légi nukleáris felderítő rendszer

továbbfejlesztése érdekében. Elsőként az ionizáló sugárzás elméleti alapjairól adunk leírást. Bemutatjuk a

sugárzás különböző típusait, azok jellemzőit, behatolási tartományaikat, és az anyagra gyakorolt hatásaikat.

Ezután egy kritikai elemzést végzünk a magyar nukleáris felderítő rendszerről, a felderítési kapacitásokra

alapozva. Az utolsó rész a hiperspektrális képalkotási technológia lehetőségeit deríti fel, annak hozzáadott értékeit

és megvizsgálandó alkalmazási lehetőségeit.

1. INTRODUCTION

Aerial nuclear reconnaissance is a survey technique used to detect, measure, identify and map

radioactivity in the environment. The main field of application is the estimation of the

contamination extent after a nuclear catastrophe or nuclear attack. Another possible application is

the localization of lost punctual radioactive sources. All the aerial nuclear reconnaissance

techniques are based on gamma radiation detection because it is the most penetrating radiation type.

In average, in order to detect moderately intense radiations, the reconnaissance is done at an altitude

of 200m above the ground. This is because of the quick attenuation of the gamma radiation by the

air. In this study we would like to explore the opportunity offered by an optical remote sensing

technique called hyperspectral imaging. Our assumption is that with optical remote sensing

technique it could be possible to detect a larger range of radiations from a higher flight altitude.

1 National University of Public Service, Doctoral School of Military Engineering, gregory.luc4s@gmail.com 2 Dr., Karoly Robert College, Institute of Remote Sensing and Rural Developement, lenart.dr@gmail.com

3 Col(Ret.) Prof. Em., National University of Public Service, Institute for Disaster Management,

solymosi.jozsef@uni-nke.hu 4 Publisher’s reader: Laszlo Pokoradi (PhD), Professor, University of Debrecen, pokoradi.laszlo@prosysmod.hu 5 Publisher’s reader: Colonel Robert Szabolcsi (PhD), Professor, National University of Public Service

Technology Department of Military Aviation, szabolcsi.robert@uni-nke.hu

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2. CHARACTERIZATION OF IONIZING RADIATIONS

Ionizing radiation is radiation composed of particles that individually carry enough energy to

liberate an electron from an atom or molecule. This involves the ejection of an orbital electron,

resulting in the creation of an ion pair.

X → X+ + e– (generic formula for single ionization)

Ionizing radiation includes cosmic rays, alpha, beta and gamma rays, X-rays, and in general

any charged particle moving at relativistic speeds. In the present study we consider ionizing

radiations generated through nuclear reactions, either artificial or natural. [1]

2.1. The different types of ionizing radiations

Radiations can be grouped into directly ionizing radiations and indirectly ionizing radiations

(Tab.1). Directly ionizing radiations include all charged particles such as alpha particles, beta

particles and heavier ions. All charged particle radiations lose energy interaction with the orbital

electrons or nuclei of atoms in the materials they traverse. Indirectly ionizing radiations include

some types of electromagnetic radiations and neutrons. These radiations interact with matter by

giving rise to secondary radiation which is ionizing. Indirectly ionizing radiations lose energy

by collisions with electrons, or atomic nuclei, and the charged particles thus set in motion

interact in turn with the orbital electrons or nuclei. [1]

Type of radiation Ionizing radiation Elementary charge Mass (MeV/c2) Electromagnetic

radiation

Indirectly ionizing ultraviolet 0 0

X ray

Gamma Ray

Particles Neutron 0 940

Directly ionizing Electron / particle β- -1 0,511

Positon / particle β+ +1 0,511

Muon -1 106

Proton +1 938

Ion 4He / particle α +2 3730

Ion 12C +6 11193

Other ions Variable Variable

Tab. 1. Main ionizing radiations and their characteristics

2.2. Penetration ranges and consequences for airborne detection

The aerial detection of ionizing radiations is governed by two main principles which are the

interaction of the ionizing radiations with matter (both with the sensor and the air) and the

penetration of the ionizing radiation. [1]

Charged particles such as electrons, positrons, protons, alpha particles and beta particles

strongly interact with electrons of an atom or molecule. Consequently they have a very low

penetration range in matter. For example Alpha particles are absorbed by about 10-2 m of air

and the penetration range of Beta particles is about 8 m in air. [1] Those radiations stopped far

before the sensor cannot “activate” crystals or induce ions pair production in the chamber of

scintillation or semi-conductor sensor used in classical airborne applications. With the present

technology and practices those radiations are lost for the detection process.

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Neutral particles like gamma and neutron interact less with matter, are indirectly ionizing and

have the higher penetration range. The penetration range of gamma radiation is several

hundreds of meters in the air. The penetration of neutron radiation depends on the content of

water of the air, as water shields neutron radiation. Neutron radiation can induce gamma

radiation after atomic activation. [1]

2.3. Effect on matter and detection strategies

Let’s first confront the two different detection strategies considered in this study: Airborne

Gamma Spectroscopy (AGS) and remote sensing optical methods.

AGS has been recognized as a very powerful tool for the detection of ground contamination and

to locate lost radioactive sources. It is presently the method recommended by the IAEA for the

detection of ionizing radiations. [2] Optical detection of radioactivity by remote sensing is still

under experiment and has not proved yet efficiency in ionization detection. [6][7]

The two methods lay on totally different measurement principles. AGS detection principle is based

on the “capture” of energetic photons by the detector, which means all the photons from gamma

radiation stopped in the air prior the sensor are lost for the detection process and the low penetration

range radiations (alpha and beta) are lost too. Individual radionuclide emits gamma rays of specific

energies that are characteristic for an element and isotope. Gamma ray measurements can be

conducted in two modes. Total count measurements register gamma rays of all energies. These are

used to monitor the gross level of the gamma radiation field and to detect the presence of anomalous

sources. Spectrometers, on the other hand, measure both the intensity and energy of radiation (Fig.

1), and this enables the source of the radiation to be diagnosed. Gamma ray spectrometry is thus a

powerful tool for monitoring the radiation environment. [2]

Fig. 1. Example of spectrogram generated by gamma spectrometry

Optical remote sensing methods can in theory detect what happens in the air and in the vicinity

of the radioactive source (which encompass the effect of alpha and beta radiations on matter) as

the detection medium (the electromagnetic radiations) is traveling almost freely in the air. The

detection principles should allow detecting electronic excitation generated by the ionizing

radiations (emission) and the molecular species generated by the ionization of the air (through the

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radiations absorbed, reflected or transmitted and the associated spectral signature).

In conclusion the two detection methods are complementary and could be used in combination

to improve the detection range and the accuracy of the detection.

3. CRITICAL DESCRIPTION OF THE HUNGARIAN NUCLEAR

RECONNAISSANCE SYSTEM

The gamma spectrometry airborne nuclear reconnaissance system was designed for the primary

survey of area contaminated by radiological materials. The system can be used to reach three

major goals:

to measure the extended contamination on a territory and to map it. This is made with

total count measure of radiation rate.

the localization of radioactive point sources.

the identification of radioactive isotopes. [3][4][5]

3.1. System built up

The container of the system includes two nuclear detectors, a GPS-receiver, a barometric

altitude-meter and data logger which can send the recorded data to the on-board notebook or to

the PC of the operation center. One of the two detectors is a Geiger Müller tube (BNS-98) dose

rate meter while the other one is a specially designed, highly sensitive NDI-65/SK type

intelligent scintillation detector, built in a lead collimator which ensures the capability of

finding and localizing discrete radiation sources on the ground. The system calculates the

radiation level of the contaminated area (referred to 1 m altitude) or the dose rate of a discrete

radiation source from the dose rate measured at the flying altitude considering the atmospheric

and ground conditions.

Fig. 2. Details about the composition of the nuclear reconnaissance system

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3.2. Characteristics of the system

Name Sensors in the system Operating mode Detection capacities

LABV

airborne

-BNS-98 (2s) remote

dose-rate meter (GM

tube);

- NaI(Tl) (0,5s) crystal

NDI-65/SK intelligent

scintillator in plumb

collimator

Helicopter (MI-24D)

uniform contamination:

Speed: 150-180 km/h

Altitude: 80-100 m.

Coverage: 300 km2/h

Point source:

Speed: 100-120 km/h

Altitude: 50-60 m.

Coverage: 18-20 km2/h

Point sources:

1,5-2 times the natural ground

value 2-3 times the natural ground

value for radiation level

uniform activities:

over 2-5 mGy/h of dose rate

over 10-20 μGy/h count rate

Tab. 2. Main characteristics of the Hungarian reconnaissance system.

The main limitations identified is the low flying altitude required to measure gamma radiation, from

50m to 200m above ground. Another limitation is the fact that low penetration radiations (which are

stopped by the air in the vicinity of the radioactive materials) cannot be detected and are lost for the

detection process. It should also be noticed that the reconnaissance system is not performing geo-

referenced imaging of the impacted area during the flight, which in case of catastrophe management

could be a source of relevant information for evaluating the impacts on environment and population.

4. PRESENTATION OF HYPER SPECTRAL IMAGING TECHNOLOGY,

WORKING PRINCIPLE AND AVAILABLE SENSOR

Hyper spectral images are produced by instruments called imaging spectrometers. The

spectrometers measure the energy received simultaneously in hundreds of narrow (several nm)),

adjacent spectral bands. These measurements make it possible to derive a continuous spectrum

for each image cell. Spectroscopy science analyzes how reflectance varies with wavelength in

a spectrum. A spectrum is like a fingerprint where are appearing spectral domains of low and

high reflectance as a consequence of the physico-chemical properties of the materials surveyed.

By the identification of characteristic absorption or reflection patterns it is possible to determine

which materials are imaged. [11]

Fig. 3. General principle of hyperspectral imaging - ©MicroImages, Inc., 1999-2012

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Image spectra can be compared with field or laboratory reflectance spectra in order to recognize

and map surface materials such as particular types of vegetation or diagnostic minerals

associated with ore deposits. Wavelength-specific absorption may also be caused by the

presence of particular chemical elements or ions or the ionic charge of certain elements.

Reflectance varies with wavelength for most materials because energy at certain wavelengths

is scattered or absorbed to different degrees. [11]

Once should distinguish the reflected light spectroscopy method where the spectral reflectance

(ratio of reflected energy to incident energy as a function of wavelength) is measured and the

emission spectroscopy where the electromagnetic emission from elements or chemical species

are measured.

The most important characteristics of imaging spectrometers are:

spectral range

spectral resolution

spatial resolution

signal-to-noise ratio

The Tab. 3. shows a summary of the airborne sensors owned by Karoly Robert College.

Sensor name Sensor type Spectral range

AISA Eagle Hyperspectral sensor (VNIR) 400-970 nm

AISA Hawk Hyperspectral sensor (SWIR) 970-2450 nm

Tab. 3. Available sensors and their detection ranges

The AISA Eagle and Hawk respectively cover the 400-970 nm and 970-2450 nm spectral range.

Their technical characteristics differ regarding the spectral resolution: 2,9 nm for Eagle and 8,5

nm for Hawk. When used at the highest spectral resolution the dual sensor collects 498 bands

in the 400-2450 nm region. Spectral binning which consist in regrouping spectral bands is

possible with the two sensors and offer a stronger signal if there is a strong response in one part

of the spectrum. The detailed specifications are provided in Tab. 4.

Tab.4. Technical specifications of the AISA dual hyperspectral sensor by SPECIM

Natural color images and RGB-IR images (orthophotos) can be derived from hyper spectral

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imaging. This additional source of information is of high relevance for catastrophe

management.

Hyperspectral imaging had demonstrated many applications in resource management,

agriculture, mineral exploration, monitoring of vegetation and contamination detection. At

present no application was attempted for the detection of ionizing radiations but some

successful applications at the margin of our topic could be adapted to fit our specific objectives.

For example some results were published on successful LWIR identification of hazardous

gasses. [16] LWIR hyperspectral imaging is also capable in identifying chemicals used in

chemical warfare (Farley et al, 2006). [17] FTIR imaging has been successfully used to identify

radioactive materials. [8] Some preliminary works has been done with hyperspectral remote

sensing for the identification of uranium mine tailings.

The work recently done for the detection of soil contamination with the red mud catastrophe in

Kolontar also shows some interesting potential with the detection of contaminant in low

concentration in soil. [13]

Last but not least, hyperspectral technology is evolving very quickly. The sensors developed

recently have a made a significant progress with signal-to-noise ratio and spectral resolution.

This open new possibilities for the detection of traces of gas and molecules. [9][14]

5. STUDY ON THEORETICAL PHYSICAL BASIS

In the previous chapter we have described the general principles regarding hyperspectral

technology and have introduced one sensor as an example. In the light of the additional

explanations given about the ionizing radiations and their interaction with matter we are trying

in this chapter to set some basis for the indirect detection of ionizing radiations with

hyperspectral imaging technology.

5.1. The strategical basis

As we have seen previously, presently the aerial detection of ionizing radiation is only done with

Aerial Gamma Spectrometry (AGS). This method specifically senses the high energy photons

generated by the decay of radiological materials (only gamma unscattered radiation). Only

photons have a sufficient penetration range to travel in the air and reach an airborne sensor. Alpha

and Beta radiations which are respectively stopped by a few cm and 9 cm of air are lost for such

detection process. Gamma radiation intensity decreases exponentially with altitude. Because of

this reason, the sensor should be flown at an average altitude of 100-200m (with helicopter) for

moderately intense radiation sources, which is a main disadvantage: it is costly and lack of

flexibility as regards to the new challenges in nuclear reconnaissance.

We would like to develop a new detection method based on a different strategy. Instead of using

gamma radiation detection, we would like to use an optical remote sensing approach and to detect

the ions and molecules specifically generated in the air by the ionization radiations around

radioactive materials. This approach would be done with reflected light spectrometry. This is an

indirect measurement method, but it offers two advantages. First the effects of alpha, beta and

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scattered gamma radiations on matter would be sensed. The activity of radioactive materials could

then be retrieved from appropriate calibration and computation. As a consequence hyperspectral

imaging could be used as a complementary method. AGS and hyperspectral imaging would detect

(indirectly for the sake of hyperspectral imaging) the full range of radiations in the vicinity of the

radiological material and in the air from the radiological source to the sensor. Secondly, as light

travel more freely in the air, the flying altitude could be increased. Puckrin have demonstrated

that in the case of passive detection with FTIR, radiation can in theory be detected from an altitude

of 1000m above the ground if the conditions are optimal. [8] The demonstration was made using

MODTRAN4 modeling. [8]

Fig. 4. and Tab. 5. emphasizes on the difference in the application of Airborne Gamma

Spectrometry and the application of hyperspectral imaging as regard to the flight altitude, the

platform used, the “objects” sensed, the medium used. It should be noticed that all the

information related to the implementation of hyperspectral imaging are only hypothetical as

this method was not put in practice yet.

Fig. 4. Comparison of Airborne Gamma Spectrometry and hyperpectral imaging for the detection of ionizing

radiations.

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Airborne Gamma Spectrometry Airborne hyper spectral imaging Detected Not detected and lost for

detection process

Potentially detected Not detected and

lost for detection

process

Vector Photons (gamma) Molecules and excited

atoms

Radiation Gamma ray from:

- unscattered gamma

- neutron activation.

Alpha, beta, scattered

gamma, neutron (partly)

and charged particles.

Partly: alpha, beta,

scattered gamma,

neutron, charged

particles.

Unscattered

Gamma (photons

of high energy not

stopped by the air).

Effect on

matter

X+,e- pairs creation on

the crystal of the

sensor.

X+,e- pairs created in the air

before the sensor.

Atomic excitation.

Atomic excitation.

Specific species created by

ionizing radiation (ions,

secondary product (O3))

-

Detection

range

On the crystals of the

sensors. Only the

photons intercepted by

the sensors are counted.

On the surface of the

radioactive material, in the

vicinity of RA material, in

the air between the ground

and the sensor.

In the field of view of

the sensor, i.e. in the

vicinity of radioactive

material in the air or on

the ground.

Over the sensors

(photons with high

energy)

Physical

effect used

in the

detection

Ionization and

scintillation created by

photons by:

- photoelectric effect,

- Compton scattering,

- pair production.

Electromagnetic

radiation emission (after

de-excitation of atomic

electrons), absorption

and reflexion (by the

product of ionization

reactions).

Tab. 5. Comparison of Airborne Gamma Spectrometry and hyperpectral imaging for the detection of ionizing

radiations.

The strategy with the use of hyperspectral technology can be twofold:

to detect the presence of products generated by ionization (molecules species) by

reflected light spectrometry. The absorption and reflection patterns of the products

specific to ionization reactions should then be known and identified.

to exploit the excitation generated by the ionizing radiations, which means to measure

the energy emitted by excited atoms when returning to ground state (O, N, H).

5.2. Detection of new molecular species by reflected light spectroscopy

From the bibliographic research we know that Moss already attempted to detected ionizing

radiations through the detection of new species generated by radiation with optical remote

sensing method. [6] If the strategy is the same as the one we want to develop, the method differs

as he used differential absorption LiDAR (DIAL) for the detection. Nevertheless Moss made

an interesting exploration regarding the specific species present in the surrounding of

radioactive materials. He has calculated the molar fraction as a function of time of the species

formed in air by irradiation with a 60 Curie source of 113Cd. The simulation was done with a

gas-phase chemical kinetics code developed at the Los Alamos National Laboratory. The results

of the simulation are very interesting. Instead of having ions as a product of the ionization of

air (which is expected primarily from ionization), the model shows that rather secondary

products are accumulated in the vicinity of the radiological materials. The model generated the

following secondary products: O3, N2O, HNO3, H2NO2, and NO2. The second important result

is the value of the molar fraction calculated by the model. They are very low, from the order of

10-5 to 10-7. The absence of ions is probably explained by their high reactivity and very quick

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life time in the air. Once the specific indicator species are theoretically known, we should

explore the detection possibilities. In this article we decided to concentrate on O3 and N2.

The classical absorption bands of ozone are the following:

the Hartley bands between 200 and 300 nanometers in the ultraviolet, with a very intense

maximum absorption at 255 nanometers. It is the strongest absorption band.

the Huggins bands, weak absorption between 320 and 360 nanometers

the Chappuis bands, a weak diffuse system between 375 and 650 nanometers in the

visible spectrum

the Wulf bands in the infrared beyond 700 nm, centered at 4,700, 9,600 and 14,100

nanometers, the latter being the most intense.

The figure bellow represents the absorption bands for ozone and the different domain of

hyperspectral sensors.

Fig. 5. Match between the absorption of ozone and the light spectrum sub-regions.

Some absorption is located in the VNIR and MWIR regions. The highest potential is in the

LWIR region comprising the 14100 nm band (Fig. 5.).

Regarding the other target species, additional research about their spectral signature should be

done in order to know if it makes sense to try to detect them.

5.3. Electronic excitation

Regarding nitrogen, the following reactions happen:

N2→ N2* N2* → N2 + hv

References are available about the atomic emission line from spectroscopy analysis. Atomic

emission spectroscopy is a method of chemical analysis used in laboratory for identifying the

elements in a sample. The principle reposes on the emission of photons by excited atoms.

Fig. 8. and 9. represent the atomic emission spectrum of Nitrogen. They were elaborated from

the atomic basic spectroscopic data provided by the National Institute of Standards and

Technology (NIST). [15] Only the observed strong emission lines are represented on the figure.

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Fig. 6. Atomic emission spectrum for Nitrogen I-II in the spectral range of Eagle (4000-9500 Å).

Several spectral regions show a high density of emission lines. This is of interest for the

detection with the hyperspectral sensors. In the spectral range of Eagle the 460, 500, 570, 750,

820, 870 and 940 nm regions seem promising.

Fig. 7. Atomic emission spectrum for Nitrogen I-II in the 9500-14 000Å region.

The emission spectrum of nitrogen in the spectral range of Hawk seems quantitatively less

important. Emission lines in the 1010 and 1350 nm region could offer some detection

possibilities.

The excited state of oxygen is somewhat more stable than nitrogen. While de-excitation can

occur by emission of photons, more probable mechanism at atmospheric pressure is a chemical

reaction with other oxygen molecules, forming ozone.

O2 → O2+ + e–

O2+ + 2O2 → 2O3

The detection possibilities are then the same as the ones already exposed above. The creation

of ozone molecule by both excitation and ionization is an interesting fact for the detection

capacities as it can strengthen the absorption signal.

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5.4. Limitations and problem expected with hyperspectral imaging

As mentioned, only traces of molecular species are expected in the air around the radiological

materials. We wonder about the intensity of the spectral signature the sensor can detect from

the specific species. The question of the relative intensity compared to the other spectral

signatures (background) is a key point for the extraction of the spectrum of the specific species.

If the reflectance is too weak, the atmosphere could also create too much disturbances and their

spectrum could not be extractable. A last question is the spectral accuracy necessary to be sure

to see the spectral signature. Is the scale of 3-4 nm sufficient or should the sensor have a sub-

nanometer spectral resolution? Laboratory and field test with different sensors will try to answer

these questions.

6. CONCLUSIONS AND PERSPECTIVES

The theoretical analysis conducted on hyperspectral technology reveals an interesting potential

for the detection of alpha, beta and scattered gamma radiations through the detection of specific

signal signatures from the generated ions and secondary products of ionization reactions.

Consequently hyperspectral imaging potentially constitute a complementary detection method

to aerial gamma spectrometry. The integration of the two detection methods on the same

platform would allow an integrated approach in the detection of ionizing radiations.

Furthermore this optical remote sensing technique can be applied at an altitude much higher

than 100m. Nevertheless several difficulties have been identified. It seems the products of

ionization are present in the air in infinitesimal quantities. Laboratory and field measurements

work should confirm if optical detection is sensitive enough and applicable. The strength of the

radiological source and the distance with the source are important parameters to consider in this

work. A second question deals with the reflectance and spectral signature of ionized molecules

and secondary products, in particular the intensity and profile of the signals. A weak or “flat”

signal would offer limited applications as the spectral signature could not be extracted from a

mixed spectrum comprising the general environmental effects (atmospheric and background).

The laboratory work in the near future should help to determine which spectral region is the

most promising and which indicator species have the best potential for ionizing radiation

detection.

Hyper spectral imaging offers two other added values. Orthorectified RGB-IR images can be

produced and used for estimating the damage on the environment. The spectral signature of

vegetation in the infrared red region can be used as an indicator of stress and help in the

identification of radiological contamination.

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